The functional features of fish gonadotropins have remained unclear. This situation can be ascribed largely to the difficulty of obtaining sufficient quantities of purified gonadotropins, since many animals are needed for the collection of pituitaries and purification of gonadotropins . In addition, the structural and chemical properties between Fsh and Lh are similar, which makes it difficult to separate them and produce a specific antiserum. Consequently, it has not been possible to distinguish the two hormone-producing cells by immunohistology in many families of teleosts. Recombinant gonadotropins should be a useful tool to resolve these problems.
At present, abundant biologically active recombinant mammalian gonadotropins or their analogs have been successfully expressed in different expression systems, including yeast (Pichia pastoris), insect cells, and mammalian cells. The Bac-to-Bac baculovirus expression system provides a rapid and efficient method to generate recombinant protein, and the baculovirus system has become one of the most versatile and powerful eukaryotic vector systems for recombinant protein expression. Since 1985, when the first protein (IL-2) was produced on a large scale from a recombinant baculovirus, use of the baculovirus expression system has increased dramatically; more than 600 recombinant genes have been expressed in baculoviruses to date.